Wafer producing method

ABSTRACT

A hexagonal single crystal wafer is produced from a hexagonal single crystal ingot. The depth of the focal point of a laser beam is gradually changed from a shallow position not reaching the depth corresponding to the desired thickness of the wafer to a deep position corresponding to the desired thickness of the wafer in such a manner that a parabola is described by the path of the focal point. When the spot area of the laser beam on the upper surface of the ingot becomes a predetermined maximum value, the deep position of the focal point is maintained.

BACKGROUND OF THE INVENTION

Field of the Invention

The present invention relates to a wafer producing method for slicing a hexagonal single crystal ingot to produce a wafer.

Description of the Related Art

Various devices such as ICs and LSIs are formed by forming a functional layer on the front side of a wafer formed of silicon or the like and partitioning this functional layer into a plurality of regions along a plurality of crossing division lines. The division lines of the wafer are processed by a processing apparatus such as a cutting apparatus and a laser processing apparatus to thereby divide the wafer into a plurality of individual device chips corresponding to the respective devices. The device chips thus obtained are widely used in various equipment such as mobile phones and personal computers. Further, power devices or optical devices such as LEDs and LDs are formed by forming a functional layer on the front side of a wafer formed of a hexagonal single crystal such as SiC and GaN and partitioning this functional layer into a plurality of regions along a plurality of crossing division lines.

In general, the wafer on which the devices are to be formed is produced by slicing an ingot with a wire saw. Both sides of the wafer obtained above are polished to a mirror finish (see Japanese Patent Laid-open No. 2000-94221, for example). This wire saw is configured in such a manner that a single wire such as a piano wire having a diameter of approximately 100 to 300 μm is wound around many grooves formed on usually two to four guide rollers to form a plurality of cutting portions spaced in parallel with a given pitch. The wire is operated to run in one direction or opposite directions, thereby slicing the ingot into a plurality of wafers.

However, when the ingot is cut by the wire saw and both sides of each wafer are polished to obtain the product, 70 to 80% of the ingot is discarded to cause a problem of poor economy. In particular, a hexagonal single crystal ingot of SiC or GaN, for example, has high Mohs hardness and it is therefore difficult to cut this ingot with the wire saw. Accordingly, considerable time is required for cutting of the ingot, causing a reduction in productivity. That is, there is a problem in efficiently producing a wafer in this prior art.

A technique for solving this problem is described in Japanese Patent Laid-open No. 2013-49161. This technique includes the steps of setting the focal point of a laser beam having a transmission wavelength to SiC inside a hexagonal single crystal ingot, next applying the laser beam to the ingot as scanning the laser beam on the ingot to thereby form a modified layer and cracks in a separation plane inside the ingot, and next applying an external force to the ingot to thereby break the ingot along the separation plane where the modified layer and the cracks are formed, thus separating a wafer from the ingot. In this technique, the laser beam is scanned spirally or linearly along the separation plane so that a first application point of the laser beam and a second application point of the laser beam nearest to the first application point have a predetermined positional relation with each other. As a result, the modified layer and the cracks are formed at very high density in the separation plane of the ingot.

SUMMARY OF THE INVENTION

However, in the ingot cutting method described in Japanese Patent Laid-open No. 2013-49161 mentioned above, the laser beam is scanned spirally or linearly on the ingot. In the case of linearly scanning the laser beam, the direction of scanning of the laser beam is not specified. In the ingot cutting method described in Japanese Patent Laid-open No. 2013-49161, the pitch (spacing) between the first application point and the second application point of the laser beam as mentioned above is set to 1 to 10 μm. This pitch corresponds to the pitch of the cracks extending from the modified layer along a c-plane defined in the ingot.

In this manner, the pitch of the application points of the laser beam to be applied to the ingot is very small. Accordingly, regardless of whether the laser beam is scanned spirally or linearly, the laser beam must be applied with a very small pitch and the improvement in productivity is not yet sufficient.

It is therefore an object of the present invention to provide a wafer producing method which can efficiently produce a wafer from an ingot.

In accordance with an aspect of the present invention, there is provided a wafer producing method for producing a hexagonal single crystal wafer from a hexagonal single crystal ingot having a first surface, a second surface opposite to the first surface, a c-axis extending from the first surface to the second surface, and a c-plane perpendicular to the c-axis, the wafer producing method including a separation start point forming step of setting the focal point of a laser beam having a transmission wavelength to the ingot inside the ingot at a predetermined depth from the first surface, which depth corresponds to the thickness of the wafer to be produced, and next applying the laser beam to the first surface as relatively moving the focal point and the ingot to thereby form a modified layer parallel to the first surface and cracks extending from the modified layer along the c-plane, thus forming a separation start point, and a wafer separating step of separating a plate-shaped member having a thickness corresponding to the thickness of the wafer from the ingot at the separation start point after performing the separation start point forming step, thus producing the wafer from the ingot, the separation start point forming step including a modified layer forming step of relatively moving the focal point of the laser beam in a first direction perpendicular to a second direction where the c-axis is inclined by an off angle with respect to a normal to the first surface and the off angle is formed between the first surface and the c-plane, thereby linearly forming the modified layer extending in the first direction, and an indexing step of relatively moving the focal point in the second direction to thereby index the focal point by a predetermined amount, wherein in making the focal point of the laser beam enter the ingot in the modified layer forming step, the depth of the focal point is gradually changed from a shallow position not reaching the depth corresponding to the thickness of the wafer to a deep position corresponding to the thickness of the wafer in such a manner that a parabola is described by the path of the focal point, and when the spot area of the laser beam on the first surface becomes a predetermined maximum value, the deep position of the focal point is maintained.

According to the wafer producing method of the present invention, each modified layer is formed at the predetermined depth from the first surface of the ingot and the cracks are formed on both sides of each modified layer so as to propagate along the c-plane. Accordingly, any adjacent ones of the plural modified layers are connected together through the cracks formed therebetween, so that the plate-shaped member having the thickness corresponding to the thickness of the wafer can be easily separated from the ingot at the separation start point, thus producing the hexagonal single crystal wafer from the ingot. Accordingly, the productivity can be sufficiently improved and the amount of the ingot to be discarded can be sufficiently reduced to approximately 30%.

In forming a modified layer inside the ingot by applying a laser beam having a transmission wavelength to the ingot, it is known that the modified layer is formed at a position above the focal point of the laser beam where the power density (average power/spot area) of the laser beam becomes 6.8×10⁴ W/mm².

Accordingly, at the initial stage where the focal point of the laser beam has entered the ingot from the outer circumference thereof, the power density becomes 6.8×10⁴ W/mm² at the focal point, and the modified layer is therefore formed at the focal point. Thereafter, the spot area of the laser beam on the first surface of the ingot is gradually increased, so that the power density becomes 6.8×10⁴ W/mm² at a position above the focal point. That is, the modified layer is formed at this position above the focal point. Accordingly, the path of the modified layer describes a parabola from a position far from the first surface toward the first surface. When the path of the modified layer reaches a predetermined position at a given depth from the first surface, the depth of the modified layer becomes stable. Accordingly, in the conventional applying method for the laser beam, the plural elements of the linear modified layer cannot be formed in the same plane.

To solve this problem, the modified layer forming step in the present invention is improved in the following manner. At the initial stage where the focal point of the laser beam has entered the ingot, the depth of the focal point is gradually changed from a shallow position not reaching the depth corresponding to the thickness of the wafer to a deep position corresponding to the thickness of the wafer in such a manner that a parabola as canceling the above parabola described by the path of the modified layer is described by the path of the focal point, and when the spot area of the laser beam on the first surface of the ingot becomes a predetermined maximum value, the deep position of the focal point is maintained. Accordingly, the plural elements of the linear modified layer can be formed in the same plane.

The above and other objects, features and advantages of the present invention and the manner of realizing them will become more apparent, and the invention itself will best be understood from a study of the following description and appended claims with reference to the attached drawings showing a preferred embodiment of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of a laser processing apparatus suitable for use in performing a wafer producing method of the present invention;

FIG. 2 is a block diagram of a laser beam generating unit;

FIG. 3A is a perspective view of a hexagonal single crystal ingot;

FIG. 3B is an elevational view of the ingot shown in FIG. 3A;

FIG. 4 is a perspective view for illustrating a separation start point forming step;

FIG. 5 is a plan view of the ingot shown in FIG. 3A;

FIG. 6 is a schematic sectional view for illustrating a modified layer forming step;

FIG. 7 is a schematic plan view for illustrating the modified layer forming step;

FIGS. 8A and 8B are schematic sectional views for illustrating a conventional positioning method for the focal point of a laser beam;

FIGS. 9A and 9B are schematic sectional views for illustrating a positioning method for the focal point of a laser beam according to the present invention;

FIGS. 10A and 10B are perspective views for illustrating a wafer separating step; and

FIG. 11 is a perspective view of a hexagonal single crystal wafer produced from the ingot.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

A preferred embodiment of the present invention will now be described in detail with reference to the drawings. Referring to FIG. 1, there is shown a perspective view of a laser processing apparatus 2 suitable for use in performing a wafer producing method of the present invention. The laser processing apparatus 2 includes a stationary base 4 and a first slide block 6 mounted on the stationary base 4 so as to be movable in the X direction. The first slide block 6 is moved in a feeding direction, or in the X direction along a pair of guide rails 14 by a feeding mechanism 12 composed of a ball screw 8 and a pulse motor 10.

A second slide block 16 is mounted on the first slide block 6 so as to be movable in the Y direction. The second slide block 16 is moved in an indexing direction, or in the Y direction along a pair of guide rails 24 by an indexing mechanism 22 composed of a ball screw 18 and a pulse motor 20. A support table 26 is mounted on the second slide block 16. The support table 26 is movable in the X direction and the Y direction by the feeding mechanism 12 and the indexing mechanism 22 and also rotatable by a motor stored in the second slide block 16.

A column 28 is provided on the stationary base 4 so as to project upward therefrom. A laser beam applying mechanism (laser beam applying means) 30 is mounted on the column 28. The laser beam applying mechanism 30 is composed of a casing 32, a laser beam generating unit 34 (see FIG. 2) stored in the casing 32, and focusing means (laser head) 36 mounted on the front end of the casing 32. An imaging unit 38 having a microscope and a camera is also mounted on the front end of the casing 32 so as to be aligned with the focusing means 36 in the X direction.

As shown in FIG. 2, the laser beam generating unit 34 includes a laser oscillator 40 for generating a pulsed laser beam such as YAG laser and YVO4 laser, repetition frequency setting means 42 for setting the repetition frequency of the pulsed laser beam to be generated from the laser oscillator 40, pulse width adjusting means 44 for adjusting the pulse width of the pulsed laser beam to be generated from the laser oscillator 40, and power adjusting means 46 for adjusting the power of the pulsed laser generated from the laser oscillator 40. Although especially not shown, the laser oscillator 40 has a Brewster window, so that the laser beam generated from the laser oscillator 40 is a laser beam of linearly polarized light. After the power of the pulsed laser beam is adjusted to a predetermined power by the power adjusting means 46 of the laser beam generating unit 34, the pulsed laser beam is reflected by a mirror 48 included in the focusing means 36 and next focused by a focusing lens 50 included in the focusing means 36. The focusing lens 50 is positioned so that the pulsed laser beam is focused inside a hexagonal single crystal ingot 11 as a workpiece fixed to the support table 26.

Referring to FIG. 3A, there is shown a perspective view of the hexagonal single crystal ingot 11 as a workpiece to be processed. FIG. 3B is an elevational view of the hexagonal single crystal ingot 11 shown in FIG. 3A. The hexagonal single crystal ingot (which will be hereinafter referred to also simply as ingot) 11 is selected from a SiC single crystal ingot or a GaN single crystal ingot. The ingot 11 has a first surface (upper surface) 11 a and a second surface (lower surface) 11 b opposite to the first surface 11 a. The first surface 11 a of the ingot 11 is preliminarily polished to a mirror finish because the laser beam is applied to the first surface 11 a.

The ingot 11 has a first orientation flat 13 and a second orientation flat 15 perpendicular to the first orientation flat 13. The length of the first orientation flat 13 is set greater than the length of the second orientation flat 15. The ingot 11 has a c-axis 19 inclined by an off angle α toward the second orientation flat 15 with respect to a normal 17 to the upper surface 11 a and also has a c-plane 21 perpendicular to the c-axis 19. The c-plane 21 is inclined by the off angle α with respect to the upper surface 11 a. In general, in the hexagonal single crystal ingot 11, the direction perpendicular to the direction of extension of the shorter second orientation flat 15 is the direction of inclination of the c-axis 19. The c-plane 21 is set in the ingot 11 innumerably at the molecular level of the ingot 11. In this preferred embodiment, the off angle α is set to 4°. However, the off angle α is not limited to 4° in the present invention. For example, the off angle α may be freely set in the range of 1° to 6° in manufacturing the ingot 11.

Referring again to FIG. 1, a column 52 is fixed to the left side of the stationary base 4. The column 52 is formed with a vertically elongated opening 53, and a pressing mechanism 54 is vertically movably mounted to the column 52 so as to project from the opening 53.

As shown in FIG. 4, the ingot 11 is fixed to the upper surface of the support table 26 by using a wax or adhesive in the condition where the second orientation flat 15 of the ingot 11 becomes parallel to the X direction. In other words, as shown in FIG. 5, the direction of formation of the off angle α is shown by an arrow Y1. That is, the direction of the arrow Y1 is the direction where the intersection 19 a between the c-axis 19 and the upper surface 11 a of the ingot 11 is present with respect to the normal 17 to the upper surface 11 a. Further, the direction perpendicular to the direction of the arrow Y1 is shown by an arrow A. Then, the ingot 11 is fixed to the support table 26 in the condition where the direction of the arrow A becomes parallel to the X direction.

Accordingly, the laser beam is scanned in the direction of the arrow A perpendicular to the direction of the arrow Y1, or the direction of formation of the off angle α. In other words, the direction of the arrow A perpendicular to the direction of the arrow Y1 where the off angle α is formed is defined as the feeding direction of the support table 26.

In the wafer producing method of the present invention, it is important that the scanning direction of the laser beam to be applied from the focusing means 36 is set to the direction of the arrow A perpendicular to the direction of the arrow Y1 where the off angle α of the ingot 11 is formed. That is, it was found that by setting the scanning direction of the laser beam to the direction of the arrow A as mentioned above in the wafer producing method of the present invention, cracks propagating from a modified layer formed inside the ingot 11 by the laser beam extend very long along the c-plane 21.

In performing the wafer producing method according to this preferred embodiment, a separation start point forming step is performed in such a manner that the focal point of the laser beam having a transmission wavelength (e.g., 1064 nm) to the hexagonal single crystal ingot 11 fixed to the support table 26 is set inside the ingot 11 at a predetermined depth from the first surface (upper surface) 11 a, which depth corresponds to the thickness of a wafer to be produced, and the laser beam is next applied to the upper surface 11 a as relatively moving the focal point and the ingot 11 to thereby form a modified layer 23 parallel to the upper surface 11 a and cracks 25 propagating from the modified layer 23 along the c-plane 21, thus forming a separation start point (separation plane) where the modified layer 23 and the cracks 25 are formed.

This separation start point forming step includes a modified layer forming step of relatively moving the focal point of the laser beam in the direction of the arrow A perpendicular to the direction of the arrow Y1 where the c-axis 19 is inclined by the off angle α with respect to the normal 17 to the upper surface 11 a and the off angle α is formed between the c-plane 21 and the upper surface 11 a, thereby forming the modified layer 23 inside the ingot 11 and the cracks 25 propagating from the modified layer 23 along the c-plane 21, and also includes an indexing step of relatively moving the focal point in the direction of formation of the off angle α, i.e., in the Y direction to thereby index the focal point by a predetermined amount as shown in FIG. 7 and FIGS. 8A and 8B.

As shown in FIGS. 6 and 7, the modified layer 23 is linearly formed so as to extend in the X direction, so that the cracks 25 propagate from the modified layer 23 in opposite directions along the c-plane 21. In the wafer producing method according to this preferred embodiment, the separation start point forming step further includes an index amount setting step of measuring the width of the cracks 25 formed on one side of the modified layer 23 along the c-plane 21 and then setting the index amount of the focal point according to the width measured above. More specifically, letting W1 denote the width of the cracks 25 formed on one side of the modified layer 23 so as to propagate from the modified layer 23 along the c-plane 21, the index amount W2 of the focal point is set in the range of W1 to 2W1.

For example, the separation start point forming step is performed under the following laser processing conditions.

Light source: Nd:YAG pulsed laser

Wavelength: 1064 nm

Repetition frequency: 80 kHz

Average power: 3.2 W

Pulse width: 4 ns

Spot diameter: 3 μm

Numerical aperture (NA) of the focusing lens: 0.43

Index amount: 250 to 400 μm

Work feed speed: 120 to 260 mm/second

In the laser processing conditions mentioned above, the width W1 of the cracks 25 propagating from the modified layer 23 along the c-plane 21 in one direction as viewed in FIG. 6 is set to approximately 250 μm, and the index amount W2 is set to 400 μm. However, the average power of the laser beam is not limited to 3.2 W. When the average power of the laser beam was set to 2 W to 4.5 W, good results were obtained in the preferred embodiment. In the case that the average power was set to 2 W, the width W1 of the cracks 25 was approximately 100 μm. In the case that the average power was set to 4.5 W, the width W1 of the cracks 25 was approximately 350 μm.

In the case that the average power is less than 2 W or greater than 4.5 W, the modified layer 23 cannot be well formed inside the ingot 11. Accordingly, the average power of the laser beam to be applied is preferably set in the range of 2 W to 4.5 W. For example, the average power of the laser beam to be applied to the ingot 11 was set to 3.2 W in this preferred embodiment. As shown in FIG. 6, the depth D1 of the focal point from the upper surface 11 a in forming the modified layer 23 was set to 500 μm.

Referring to FIG. 8A, there is shown a schematic sectional view for illustrating a conventional positioning method for the focal point of a laser beam. FIG. 8B is an enlarged view of a part shown in FIG. 8A. It is known that a modified layer is formed inside a workpiece at a position above the focal point of the laser beam where the power density (average power/spot area) of the laser beam becomes 6.8×10⁴ W/mm². Accordingly, at the initial stage where the focal point of the laser beam has entered the ingot 11 from the outer circumference (side surface) thereof, the power density becomes 6.8×10⁴ W/mm² at the focal point P, and the modified layer 23 is therefore formed at the focal point P. Thereafter, the spot area of the laser beam on the first surface (upper surface) 11 a of the ingot 11 is gradually increased, so that the power density becomes 6.8×10⁴ W/mm² at a position above the focal point P. That is, the modified layer 23 is formed at this position above the focal point P. Accordingly, the path of the modified layer 23 describes a parabola from a position far from the upper surface 11 a toward the upper surface 11 a. When the path of the modified layer 23 reaches a predetermined position at a given depth from the upper surface 11 a, the depth of the modified layer 23 becomes stable.

Letting H1 denote the height of this predetermined position from the focal point P and D2 denote the distance from the side surface of the ingot 11 to this predetermined position, H1=15 to 25 μm and D2=20 to 30 μm. Accordingly, in the conventional forming method for the focal point, the plural elements of the linear modified layer 23 cannot be formed at the same depth from the upper surface 11 a of the ingot 11, i.e., cannot be formed in the same plane.

To solve this problem, the wafer producing method according to the present invention has improved the forming method for the focal point as shown in FIGS. 9A and 9B. In making the focal point P of the laser beam enter the ingot 11, the depth of the focal point P is gradually changed from a shallow position not reaching the depth corresponding to the thickness of the wafer to a deep position corresponding to the thickness of the wafer in such a manner that a parabola is described by the path of the focal point P, and when the spot area of the laser beam on the first surface (upper surface) 11 a becomes a predetermined maximum value, the deep position of the focal point P is maintained. The vertical movement of the focal point P is controlled by the fine movement of the focusing means 36. By controlling the depth of the focal point P as shown in FIG. 9A, the plural elements of the linear modified layer 23 can be formed in the same plane as shown in FIG. 9B.

In this manner, the focal point of the laser beam is sequentially indexed to form a plurality of modified layers 23 at the depth D1 in the whole area of the ingot 11 and the cracks 25 extending from each modified layer 23 along the c-plane 21 as shown in FIGS. 6 and 7. Thereafter, a wafer separating step is performed in such a manner that an external force is applied to the ingot 11 to thereby separate a plate-shaped member having a thickness corresponding to the thickness of the wafer to be produced, from the ingot 11 at the separation start point composed of the modified layers 23 and the cracks 25, thus producing a hexagonal single crystal wafer 27 shown in FIG. 11.

This wafer separating step is performed by using the pressing mechanism 54 shown in FIG. 1. The configuration of the pressing mechanism 54 is shown in FIGS. 10A and 10B. The pressing mechanism 54 includes a head 56 vertically movable by a moving mechanism (not shown) incorporated in the column 52 shown in FIG. 1 and a pressing member 58 rotatable in the direction shown by an arrow R in FIG. 10B with respect to the head 56. As shown in FIG. 10A, the pressing mechanism 54 is relatively positioned above the ingot 11 fixed to the support table 26. Thereafter, as shown in FIG. 10B, the head 56 is lowered until the pressing member 58 comes into pressure contact with the upper surface 11 a of the ingot 11.

In the condition where the pressing member 58 is in pressure contact with the upper surface 11 a of the ingot 11, the pressing member 58 is rotated in the direction of the arrow R to thereby generate a torsional stress in the ingot 11. As a result, the ingot 11 is broken at the separation start point where the modified layers 23 and the cracks 25 are formed. Accordingly, the hexagonal single crystal wafer 27 shown in FIG. 11 can be separated from the hexagonal single crystal ingot 11. After separating the wafer 27 from the ingot 11, the separation surface of the wafer 27 and the separation surface of the ingot 11 are preferably polished to a mirror finish.

The present invention is not limited to the details of the above described preferred embodiment. The scope of the invention is defined by the appended claims and all changes and modifications as fall within the equivalence of the scope of the claims are therefore to be embraced by the invention. 

What is claimed is:
 1. A wafer producing method for producing a hexagonal single crystal wafer from a hexagonal single crystal ingot having a first surface, a second surface opposite to said first surface, a c-axis extending from said first surface to said second surface, and a c-plane perpendicular to said c-axis, said wafer producing method comprising: a separation start point forming step of setting the focal point of a laser beam having a transmission wavelength to said ingot inside said ingot at a predetermined depth from said first surface, which depth corresponds to the thickness of said wafer to be produced, and next applying said laser beam to said first surface as relatively moving said focal point and said ingot to thereby form a modified layer parallel to said first surface and cracks extending from said modified layer along said c-plane, thus forming a separation start point; and a wafer separating step of separating a plate-shaped member having a thickness corresponding to the thickness of said wafer from said ingot at said separation start point after performing said separation start point forming step, thus producing said wafer from said ingot, said separation start point forming step including a modified layer forming step of relatively moving the focal point of said laser beam in a first direction perpendicular to a second direction where said c-axis is inclined by an off angle with respect to a normal to said first surface and said off angle is formed between said first surface and said c-plane, thereby linearly forming said modified layer extending in said first direction, and an indexing step of relatively moving said focal point in said second direction to thereby index said focal point by a predetermined amount, wherein in making the focal point of said laser beam enter said ingot in said modified layer forming step, the depth of said focal point is gradually changed from a shallow position not reaching the depth corresponding to the thickness of said wafer to a deep position corresponding to the thickness of said wafer in such a manner that a parabola is described by the path of said focal point, and when the spot area of said laser beam on said first surface becomes a predetermined maximum value, said deep position of said focal point is maintained.
 2. The wafer producing method according to claim 1, wherein said hexagonal single crystal ingot is selected from the group consisting of an SiC single crystal ingot and a GaN single crystal ingot. 